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libs/corosio/src/corosio/src/detail/select/scheduler.cpp

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libs/corosio/src/corosio/src/detail/select/scheduler.cpp
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1 //
2 // Copyright (c) 2026 Steve Gerbino
3 //
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 //
7 // Official repository: https://github.com/cppalliance/corosio
8 //
9
10 #include <boost/corosio/detail/platform.hpp>
11
12 #if BOOST_COROSIO_HAS_SELECT
13
14 #include "src/detail/select/scheduler.hpp"
15 #include "src/detail/select/op.hpp"
16 #include "src/detail/make_err.hpp"
17 #include "src/detail/posix/resolver_service.hpp"
18 #include "src/detail/posix/signals.hpp"
19
20 #include <boost/corosio/detail/except.hpp>
21 #include <boost/corosio/detail/thread_local_ptr.hpp>
22
23 #include <chrono>
24 #include <limits>
25
26 #include <errno.h>
27 #include <fcntl.h>
28 #include <sys/select.h>
29 #include <sys/socket.h>
30 #include <unistd.h>
31
32 /*
33 select Scheduler - Single Reactor Model
34 =======================================
35
36 This scheduler mirrors the epoll_scheduler design but uses select() instead
37 of epoll for I/O multiplexing. The thread coordination strategy is identical:
38 one thread becomes the "reactor" while others wait on a condition variable.
39
40 Thread Model
41 ------------
42 - ONE thread runs select() at a time (the reactor thread)
43 - OTHER threads wait on wakeup_event_ (condition variable) for handlers
44 - When work is posted, exactly one waiting thread wakes via notify_one()
45
46 Key Differences from epoll
47 --------------------------
48 - Uses self-pipe instead of eventfd for interruption (more portable)
49 - fd_set rebuilding each iteration (O(n) vs O(1) for epoll)
50 - FD_SETSIZE limit (~1024 fds on most systems)
51 - Level-triggered only (no edge-triggered mode)
52
53 Self-Pipe Pattern
54 -----------------
55 To interrupt a blocking select() call (e.g., when work is posted or a timer
56 expires), we write a byte to pipe_fds_[1]. The read end pipe_fds_[0] is
57 always in the read_fds set, so select() returns immediately. We drain the
58 pipe to clear the readable state.
59
60 fd-to-op Mapping
61 ----------------
62 We use an unordered_map<int, fd_state> to track which operations are
63 registered for each fd. This allows O(1) lookup when select() returns
64 ready fds. Each fd can have at most one read op and one write op registered.
65 */
66
67 namespace boost::corosio::detail {
68
69 namespace {
70
71 struct scheduler_context
72 {
73 select_scheduler const* key;
74 scheduler_context* next;
75 };
76
77 corosio::detail::thread_local_ptr<scheduler_context> context_stack;
78
79 struct thread_context_guard
80 {
81 scheduler_context frame_;
82
83 95 explicit thread_context_guard(
84 select_scheduler const* ctx) noexcept
85 95 : frame_{ctx, context_stack.get()}
86 {
87 95 context_stack.set(&frame_);
88 95 }
89
90 95 ~thread_context_guard() noexcept
91 {
92 95 context_stack.set(frame_.next);
93 95 }
94 };
95
96 } // namespace
97
98 120 select_scheduler::
99 select_scheduler(
100 capy::execution_context& ctx,
101 120 int)
102 120 : pipe_fds_{-1, -1}
103 120 , outstanding_work_(0)
104 120 , stopped_(false)
105 120 , shutdown_(false)
106 120 , max_fd_(-1)
107 120 , reactor_running_(false)
108 120 , reactor_interrupted_(false)
109 240 , idle_thread_count_(0)
110 {
111 // Create self-pipe for interrupting select()
112
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 120 if (::pipe(pipe_fds_) < 0)
113 ✗ detail::throw_system_error(make_err(errno), "pipe");
114
115 // Set both ends to non-blocking and close-on-exec
116
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 360 for (int i = 0; i < 2; ++i)
117 {
118
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 240 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
119
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 240 if (flags == -1)
120 {
121 ✗ int errn = errno;
122 ✗ ::close(pipe_fds_[0]);
123 ✗ ::close(pipe_fds_[1]);
124 ✗ detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
125 }
126
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 240 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
127 {
128 ✗ int errn = errno;
129 ✗ ::close(pipe_fds_[0]);
130 ✗ ::close(pipe_fds_[1]);
131 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
132 }
133
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 240 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
134 {
135 ✗ int errn = errno;
136 ✗ ::close(pipe_fds_[0]);
137 ✗ ::close(pipe_fds_[1]);
138 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
139 }
140 }
141
142
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 120 timer_svc_ = &get_timer_service(ctx, *this);
143
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 120 timer_svc_->set_on_earliest_changed(
144 timer_service::callback(
145 this,
146 2168 [](void* p) { static_cast<select_scheduler*>(p)->interrupt_reactor(); }));
147
148 // Initialize resolver service
149
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 120 get_resolver_service(ctx, *this);
150
151 // Initialize signal service
152
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 120 get_signal_service(ctx, *this);
153
154 // Push task sentinel to interleave reactor runs with handler execution
155 120 completed_ops_.push(&task_op_);
156 120 }
157
158 240 select_scheduler::
159 120 ~select_scheduler()
160 {
161
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 120 if (pipe_fds_[0] >= 0)
162 120 ::close(pipe_fds_[0]);
163
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 120 if (pipe_fds_[1] >= 0)
164 120 ::close(pipe_fds_[1]);
165 240 }
166
167 void
168 120 select_scheduler::
169 shutdown()
170 {
171 {
172
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 120 std::unique_lock lock(mutex_);
173 120 shutdown_ = true;
174
175
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 240 while (auto* h = completed_ops_.pop())
176 {
177
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 120 if (h == &task_op_)
178 120 continue;
179 ✗ lock.unlock();
180 ✗ h->destroy();
181 ✗ lock.lock();
182 120 }
183 120 }
184
185 120 outstanding_work_.store(0, std::memory_order_release);
186
187
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 120 if (pipe_fds_[1] >= 0)
188 120 interrupt_reactor();
189
190 120 wakeup_event_.notify_all();
191 120 }
192
193 void
194 2376 select_scheduler::
195 post(capy::coro h) const
196 {
197 struct post_handler final
198 : scheduler_op
199 {
200 capy::coro h_;
201
202 explicit
203 2376 post_handler(capy::coro h)
204 2376 : h_(h)
205 {
206 2376 }
207
208 4752 ~post_handler() = default;
209
210 2376 void operator()() override
211 {
212 2376 auto h = h_;
213
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 2376 delete this;
214
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 2376 h.resume();
215 2376 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 2376 auto ph = std::make_unique<post_handler>(h);
224 2376 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 2376 std::unique_lock lock(mutex_);
227 2376 completed_ops_.push(ph.release());
228
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 2376 wake_one_thread_and_unlock(lock);
229 2376 }
230
231 void
232 165289 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 165289 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 165289 std::unique_lock lock(mutex_);
238 165289 completed_ops_.push(h);
239
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 165289 wake_one_thread_and_unlock(lock);
240 165289 }
241
242 void
243 2578 select_scheduler::
244 on_work_started() noexcept
245 {
246 2578 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 2578 }
248
249 void
250 2572 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 5144 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 ✗ stop();
255 2572 }
256
257 bool
258 167524 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 167524 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 167369 if (c->key == this)
263 167369 return true;
264 155 return false;
265 }
266
267 void
268 1 select_scheduler::
269 stop()
270 {
271 1 bool expected = false;
272
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 1 if (stopped_.compare_exchange_strong(expected, true,
273 std::memory_order_release, std::memory_order_relaxed))
274 {
275 // Wake all threads so they notice stopped_ and exit
276 {
277
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 1 std::lock_guard lock(mutex_);
278 1 wakeup_event_.notify_all();
279 1 }
280
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 1 interrupt_reactor();
281 }
282 1 }
283
284 bool
285 1 select_scheduler::
286 stopped() const noexcept
287 {
288 1 return stopped_.load(std::memory_order_acquire);
289 }
290
291 void
292 34 select_scheduler::
293 restart()
294 {
295 34 stopped_.store(false, std::memory_order_release);
296 34 }
297
298 std::size_t
299 89 select_scheduler::
300 run()
301 {
302
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 89 if (stopped_.load(std::memory_order_acquire))
303 ✗ return 0;
304
305
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 178 if (outstanding_work_.load(std::memory_order_acquire) == 0)
306 {
307 ✗ stop();
308 ✗ return 0;
309 }
310
311 89 thread_context_guard ctx(this);
312
313 89 std::size_t n = 0;
314
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 171770 while (do_one(-1))
315
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 171681 if (n != (std::numeric_limits<std::size_t>::max)())
316 171681 ++n;
317 89 return n;
318 89 }
319
320 std::size_t
321 ✗ select_scheduler::
322 run_one()
323 {
324 ✗ if (stopped_.load(std::memory_order_acquire))
325 ✗ return 0;
326
327 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
328 {
329 ✗ stop();
330 ✗ return 0;
331 }
332
333 ✗ thread_context_guard ctx(this);
334 ✗ return do_one(-1);
335 ✗ }
336
337 std::size_t
338 7 select_scheduler::
339 wait_one(long usec)
340 {
341
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 7 if (stopped_.load(std::memory_order_acquire))
342 ✗ return 0;
343
344
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 14 if (outstanding_work_.load(std::memory_order_acquire) == 0)
345 {
346
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 1 stop();
347 1 return 0;
348 }
349
350 6 thread_context_guard ctx(this);
351
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 6 return do_one(usec);
352 6 }
353
354 std::size_t
355 ✗ select_scheduler::
356 poll()
357 {
358 ✗ if (stopped_.load(std::memory_order_acquire))
359 ✗ return 0;
360
361 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
362 {
363 ✗ stop();
364 ✗ return 0;
365 }
366
367 ✗ thread_context_guard ctx(this);
368
369 ✗ std::size_t n = 0;
370 ✗ while (do_one(0))
371 ✗ if (n != (std::numeric_limits<std::size_t>::max)())
372 ✗ ++n;
373 ✗ return n;
374 ✗ }
375
376 std::size_t
377 ✗ select_scheduler::
378 poll_one()
379 {
380 ✗ if (stopped_.load(std::memory_order_acquire))
381 ✗ return 0;
382
383 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
384 {
385 ✗ stop();
386 ✗ return 0;
387 }
388
389 ✗ thread_context_guard ctx(this);
390 ✗ return do_one(0);
391 ✗ }
392
393 void
394 4141 select_scheduler::
395 register_fd(int fd, select_op* op, int events) const
396 {
397 // Validate fd is within select() limits
398
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 4141 if (fd < 0 || fd >= FD_SETSIZE)
399 ✗ detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
400
401 {
402
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 4141 std::lock_guard lock(mutex_);
403
404
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 4141 auto& state = registered_fds_[fd];
405
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 4141 if (events & event_read)
406 2168 state.read_op = op;
407
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 4141 if (events & event_write)
408 1973 state.write_op = op;
409
410
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 4141 if (fd > max_fd_)
411 184 max_fd_ = fd;
412 4141 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 4141 interrupt_reactor();
417 4141 }
418
419 void
420 4116 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 4116 std::lock_guard lock(mutex_);
424
425
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 4116 auto it = registered_fds_.find(fd);
426
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 4116 if (it == registered_fds_.end())
427 3997 return;
428
429
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 119 if (events & event_read)
430 119 it->second.read_op = nullptr;
431
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 119 if (events & event_write)
432 ✗ it->second.write_op = nullptr;
433
434 // Remove entry if both are null
435
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 119 if (!it->second.read_op && !it->second.write_op)
436 {
437
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 119 registered_fds_.erase(it);
438
439 // Recalculate max_fd_ if needed
440
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 119 if (fd == max_fd_)
441 {
442 118 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
443
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 118 for (auto& [registered_fd, state] : registered_fds_)
444 {
445 ✗ if (registered_fd > max_fd_)
446 ✗ max_fd_ = registered_fd;
447 }
448 }
449 }
450 4116 }
451
452 void
453 4141 select_scheduler::
454 work_started() const noexcept
455 {
456 4141 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 4141 }
458
459 void
460 171812 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 343624 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
464 {
465 // Last work item completed - wake all threads so they can exit.
466 90 std::unique_lock lock(mutex_);
467 90 wakeup_event_.notify_all();
468
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 90 if (reactor_running_ && !reactor_interrupted_)
469 {
470 ✗ reactor_interrupted_ = true;
471 ✗ lock.unlock();
472 ✗ interrupt_reactor();
473 }
474 90 }
475 171812 }
476
477 void
478 8596 select_scheduler::
479 interrupt_reactor() const
480 {
481 8596 char byte = 1;
482
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 8596 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 8596 }
484
485 void
486 167665 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 167665 if (idle_thread_count_ > 0)
490 {
491 // Idle worker exists - wake it via condvar
492 ✗ wakeup_event_.notify_one();
493 ✗ lock.unlock();
494 }
495
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 167665 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 2166 reactor_interrupted_ = true;
499 2166 lock.unlock();
500 2166 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 165499 lock.unlock();
506 }
507 167665 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 171687 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 6167 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 6167 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 6167 auto nearest = timer_svc_->nearest_expiry();
523
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 6167 if (nearest == timer_service::time_point::max())
524 35 return requested_timeout_us;
525
526 6132 auto now = std::chrono::steady_clock::now();
527
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 6132 if (nearest <= now)
528 15 return 0;
529
530
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 6117 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 12234 nearest - now).count();
532
533
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 6117 if (requested_timeout_us < 0)
534 6117 return static_cast<long>(timer_timeout_us);
535
536 ✗ return static_cast<long>((std::min)(
537 ✗ static_cast<long long>(requested_timeout_us),
538 ✗ static_cast<long long>(timer_timeout_us)));
539 }
540
541 void
542 84465 select_scheduler::
543 run_reactor(std::unique_lock<std::mutex>& lock)
544 {
545 // Calculate timeout considering timers, use 0 if interrupted
546
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 84465 long effective_timeout_us = reactor_interrupted_ ? 0 : calculate_timeout(-1);
547
548 // Build fd_sets from registered_fds_
549 fd_set read_fds, write_fds, except_fds;
550
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 1435905 FD_ZERO(&read_fds);
551
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 1435905 FD_ZERO(&write_fds);
552
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 1435905 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 84465 FD_SET(pipe_fds_[0], &read_fds);
556 84465 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 94324 for (auto& [fd, state] : registered_fds_)
560 {
561
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 9859 if (state.read_op)
562 7886 FD_SET(fd, &read_fds);
563
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 9859 if (state.write_op)
564 {
565 1973 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 1973 FD_SET(fd, &except_fds);
568 }
569
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 9859 if (fd > nfds)
570 7890 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 84465 struct timeval* tv_ptr = nullptr;
576
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 84465 if (effective_timeout_us >= 0)
577 {
578 84430 tv.tv_sec = effective_timeout_us / 1000000;
579 84430 tv.tv_usec = effective_timeout_us % 1000000;
580 84430 tv_ptr = &tv;
581 }
582
583
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 84465 lock.unlock();
584
585
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 84465 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 84465 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 84465 timer_svc_->process_expired();
590
591
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 84465 if (ready < 0 && saved_errno != EINTR)
592 ✗ detail::throw_system_error(make_err(saved_errno), "select");
593
594 // Re-acquire lock before modifying completed_ops_
595
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 84465 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 84465 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 12898 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 84465 int completions_queued = 0;
606
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 84465 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 6449 std::vector<int> fds_to_check;
610
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 6449 fds_to_check.reserve(registered_fds_.size());
611
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 14369 for (auto& [fd, state] : registered_fds_)
612
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 7920 fds_to_check.push_back(fd);
613
614
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 14369 for (int fd : fds_to_check)
615 {
616
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 7920 auto it = registered_fds_.find(fd);
617
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 7920 if (it == registered_fds_.end())
618 ✗ continue;
619
620 7920 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 7920 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 7920 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 2049 auto* op = state.read_op;
629 // Claim the op by exchanging to unregistered. Both registering and
630 // registered states mean the op is ours to complete.
631 2049 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 2049 if (prev != select_registration_state::unregistered)
634 {
635 2049 state.read_op = nullptr;
636
637
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 2049 if (has_error)
638 {
639 ✗ int errn = 0;
640 ✗ socklen_t len = sizeof(errn);
641 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
642 ✗ errn = errno;
643 ✗ if (errn == 0)
644 ✗ errn = EIO;
645 ✗ op->complete(errn, 0);
646 }
647 else
648 {
649 2049 op->perform_io();
650 }
651
652 2049 completed_ops_.push(op);
653 2049 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 7920 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 1973 auto* op = state.write_op;
661 // Claim the op by exchanging to unregistered. Both registering and
662 // registered states mean the op is ours to complete.
663 1973 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 1973 if (prev != select_registration_state::unregistered)
666 {
667 1973 state.write_op = nullptr;
668
669
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 1973 if (has_error)
670 {
671 ✗ int errn = 0;
672 ✗ socklen_t len = sizeof(errn);
673 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
674 ✗ errn = errno;
675 ✗ if (errn == 0)
676 ✗ errn = EIO;
677 ✗ op->complete(errn, 0);
678 }
679 else
680 {
681 1973 op->perform_io();
682 }
683
684 1973 completed_ops_.push(op);
685 1973 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 7920 if (!state.read_op && !state.write_op)
691
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 4022 registered_fds_.erase(it);
692 }
693 6449 }
694
695
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 84465 if (completions_queued > 0)
696 {
697
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 2053 if (completions_queued == 1)
698 84 wakeup_event_.notify_one();
699 else
700 1969 wakeup_event_.notify_all();
701 }
702 84465 }
703
704 std::size_t
705 171776 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 171776 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 256241 if (stopped_.load(std::memory_order_acquire))
713 ✗ return 0;
714
715 256241 scheduler_op* op = completed_ops_.pop();
716
717
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 256241 if (op == &task_op_)
718 {
719 84554 bool more_handlers = !completed_ops_.empty();
720
721
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 84554 if (!more_handlers)
722 {
723
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 12512 if (outstanding_work_.load(std::memory_order_acquire) == 0)
724 {
725 89 completed_ops_.push(&task_op_);
726 89 return 0;
727 }
728
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 6167 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 84465 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 84465 reactor_running_ = true;
737
738
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 84465 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 84465 run_reactor(lock);
742
743 84465 reactor_running_ = false;
744 84465 completed_ops_.push(&task_op_);
745 84465 continue;
746 84465 }
747
748
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 171687 if (op != nullptr)
749 {
750
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 171687 lock.unlock();
751 171687 work_guard g{this};
752
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 171687 (*op)();
753 171687 return 1;
754 171687 }
755
756 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
757 ✗ return 0;
758
759 ✗ if (timeout_us == 0)
760 ✗ return 0;
761
762 ✗ ++idle_thread_count_;
763 ✗ if (timeout_us < 0)
764 ✗ wakeup_event_.wait(lock);
765 else
766 ✗ wakeup_event_.wait_for(lock, std::chrono::microseconds(timeout_us));
767 ✗ --idle_thread_count_;
768 84465 }
769 171776 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774